Not for shooting Nerf darts. I put it on the Nerf blowguns I made for aesthetic purposes.

You want there to be a good seal on the outside of your mouth in order to get more power. The larger opening that covers your lips completely ensures a great seal. However, it is not that big of a deal when using Nerf darts as ammo. If you were using real blowgun ammo and hunting, then the mouthpiece would definitely help out.